1. Field of the Invention
This invention relates generally to a position measuring device and more particularly to a system that uses Global Positioning System (GPS) satellites to determine the position of an object.
2. Description of Related Art
It is often desirable to obtain the position and velocity of an object such as an unmanned vessel. Such objects or vessels include, for example, towed barges, aircraft and automobiles. With respect to a towed barge, the reasons for obtaining the position and velocity are clearxe2x80x94the safety of the waterways depends on knowledge of the location, course and speed of all vessels, manned or unmanned. To this end, the U.S. Coast Guard has addressed the need for situational awareness on the waterways through the Ports and Waterways Safety System (PAWSS), Vessel Traffic Services (VTS), and the Automated Identification System (AIS) transponder. Any AIS equipped vessel returns identification, location, course and speed data through the VTS to the Vessel Traffic Center (VTC) which displays the waterway traffic situation.
Through wireless DGPS receivers the position of barges can be reported to any interrogator in range. The interrogator can be the towing vessel, another manned vessel, or the VTC itself. The towing vessel can maintain precise knowledge of the position of its barges and relay that data to the VTC. A portable, self powered wireless DGPS receiver may be placed on each barge to be tracked. The receiver must operate unattended, for at least the duration of a voyage and preferably much longer, even indefinitely. The barge environment is low in vibration and dynamics, but the unit can be expected to be dropped and mishandled during transport. The RF environment will include VHF marine band activity, urban RF noise, and marine radar transmissions. Some blockage of GPS signals below 30xc2x0 elevation could be expected, but the higher elevation satellites should be visible while the unit is operating on a barge. There will be multipath in the environment from the host barge, other barges and vessels, and shore facilities.
There are at least three current solutions available to provide a DGPS tag for a remote unpowered object such as a barge. All would require a battery system connected to existing technology such as a complete DGPS transponder, a GPS pseudorange transponder, or an RF sampling transponder.
A complete DGPS transponder consists of a GPS receiver and a DGPS beacon receiver integrated with a RF modem at the tag. The GPS receiver and the Beacon receiver must remain on continuously while the tag is deployed, waiting to be interrogated. Once the unit is interrogated, it must return a position report. This system for providing a DGPS tag for a remote unpowered object is undesirable in that all the equipment must be powered continuously. Thus the system consumes a large amount of power.
A GPS pseudorange transponder consists of a GPS receiver integrated with a RF modem. No beacon corrections are collected at the transponder. The receiver must remain on to have pseudoranges available on demand. When the unit is interrogated, it transmits the pseudoranges back to the towing vessel. DGPS corrections are collected and stored at the towing vessel, and applied to the received pseudoranges. The barge position calculation is completed at the towing vessel. This system also requires that the equipment remain powered continuously in order to provide measurements on demand.
A RF sampling transponder down converts and digitizes the L1 band signal and transmits the RF samples themselves. It does not need to remain on between interrogations. Processing the RF samples for code correlations is done entirely at the interrogator. While this application has the lowest power requirements, it must transfer large amounts of data which presents its own troubles in terms of complexity and reliability.
What has been needed and heretofore unavailable is a system with very low power requirements that is small and easy to deploy. The system should use a simple but reliable communication link and should minimize data transmission to further conserve power and reduce the possibility of transmission error.
Briefly, and in general terms, the present invention provides a system for, and a method of, determining the position and velocity of an object using a plurality of GPS signals transmitted by a plurality of GPS sources.
In a first aspect the invention is related to a communications system for determining at least one of position and velocity of an object. The system includes an interrogator and a transponder. The interrogator is remote from the object and responsive to the plurality of GPS signals. The interrogator transmits an RF signal comprising GPS source information and at least one of frequency information and time and code phase information of at least one of the GPS signals. The transponder is positioned on the object and is responsive to the RF signal transmitted by the interrogator and the plurality of GPS signals. The transponder tracks one of the plurality of GPS signals in response to the GPS source information and the at least one of frequency information and time and code phase information. The transponder generates a correlation snapshot and transmits the snapshot to the interrogator. The snapshot comprises a sampled coarse acquisition (C/A) code between the one of the plurality of GPS signals and a replica of the one of the plurality of GPS signals generated by the transponder at regular offsets of some fraction of a C/A code chip. The interrogator processes the correlation snapshot to determine the position and velocity of the transponder.
In a detailed aspect of the invention, the GPS source information comprises a pseudorandom noise (PRN) code number and the frequency information comprises at least one of a carrier frequency and a Doppler offset frequency. In another detailed aspect, the frequency information comprises the carrier frequency of the RF signal transmitted by the interrogator and the transponder comprises a local oscillator that is phase-locked, through a phase lock loop, to the carrier frequency. In other detailed facets of the invention the time and code phase information comprises an offset measurement in chips, the correlation snapshot includes a set of fixed-point correlator sums and a range of offset in chips and the correlation snapshot is obtained as the set of correlator outputs summed over an integration interval.
In a second aspect the invention involves a method for determining at least one of position and velocity of an object using a plurality of GPS signals transmitted by a plurality of GPS sources. The method includes the step of positioning an interrogator remote from the object. The interrogator being responsive to the plurality of GPS signals for determining GPS source information and at least one of frequency information and time and code phase information of at least one of the GPS signals. The method also includes the step of transmitting an RF signal comprising the GPS source information and the at least one of frequency information and time and code phase information of at least one of the GPS signals from the interrogator. Also included is the step of positioning a transponder comprising a plurality of correlators on the object. The transponder being responsive to the RF signal and the plurality of GPS signals for: tracking one of the plurality of GPS signals in response to the GPS source information and the at least one of frequency information and time and code phase information, generating a correlation snapshot and transmitting the snapshot to the interrogator. The snapshot comprises a sampled coarse acquisition (C/A) code between the one of the plurality of GPS signals and a replica of the one of the plurality of GPS signals generated by the transponder at regular offsets of some fraction of a C/A code chip. The method further includes the step of processing the correlation snapshot to determine at least one of position and velocity of the transponder.
In a detailed facet of the invention, the step of generating the correlation snapshot comprises the steps of obtaining a noncoherent sum of a plurality of integrations using the plurality of correlators spaced one chip apart; determining the approximate signal peak from the noncoherent sum; prepositioning the correlators at a code phase predicted from the signal peak and performing an integration with one-eighth chip correlator spacing to produce a plurality of correlator sums. In more detailed aspects the noncoherent sum is obtained by two, one millisecond integrations using twelve correlators spaced one chip apart and the plurality of correlator sums is produced by a ten millisecond integration.
Other features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.